English abstract
Pythium-induced diseases and drought are two challenges limiting vegetable production in Oman. In this thesis entitled: "The role of endophytic fungi in suppressing damping-off diseases and promoting drought tolerance in selected
vegetable crops" endophytic fungi were isolated from native desert plants, which naturally live under extreme stressful conditions such as drought and heat, and were used to mitigate the biotic and abiotic factors in crop plants, particularly tomatoes.
The three main objectives of this study were to investigate: (1) fungal diversity in desert plants in Oman; (2) the ability of the isolated fungi in suppressing Pythium- induced damping-off of vegetable crops; and (3) the ability of fungal isolates in
improving drought tolerance.
The first part of this study focused on isolation and identification of endophytic fungi from native desert plants. The identified endophytes were Aspergillus terreus from Rhazya stricta and Tephrosia apollinea desert plants and Talaromyces variabilis
from R. stricta and Zygophyllum coccineum desert plants. In addition, the study identified two novel fungal species in Oman, Cladosporium omanense sp. nov. and Talaromyces omanensis. C. omanense was obtained from Z. coccineum, and its
identity was confirmed based on morphological characteristics as well as phylogenetic analysis based on the combined internal transcribed spacer (ITS), partial translation elongation factor 1-alpha (TEF) and partial actin (ACT) sequence data. C.
omanense differed from its phylogenetically related species C. endophyticum by its fast growing colonies and in having longer macronematous and micronematous conidiophores, thick-walled conidia with conspicuously protuberant shrivelling
surface ornamentation. The second new species, T. omanensis was isolated from the desert plant R. stricta. The new species forms a distinct sister clade to the section Subinflati in the phylogeny of internal transcribed spacer (ITS), β-tubulin (benA) and
Calmodulin (CaM). This fungus remains distinct and unique from the other species in the section Subinflati in having monoverticillate conidiophores.
The second part of the study investigated the use of the identified endophytes in biological control against Pythium aphanidermatum. Aspergillus terreus (isolates 65P and 9F) inhibited the growth of P. aphanidermatum in culture and caused
abnormal growth and loss of internal content of P. aphanidermatum hyphae. The culture filtrate (CF) of A. terreus resulted in significant rise in cellular leakage of P.
aphanidermatum mycelium and reduced spore production. A. terreus also produced glucanase enzyme and siderophore. This suggests that the cell walls of Pythium, which consist of glucan, are affected by the glucanase enzyme produced by A.
terreus. The siderophore is suggested to be involved in the inhibition of Pythium growth. In bioassay tests, the two isolates increased the survival rate of cucumber seedlings. In addition, they did not have any negative effects on the growth of
cucumber seedlings.
The study also investigated the ability of T. variabilis in interfering with P. aphanidermatum growth and suppressing Pythium-induced damping-off of cucumbers and tomatoes. T. variabilis inhibited the in vitro growth of P.
aphanidermatum and caused abnormalities in P. aphanidermatum hyphae. Extracts from T. variabilis induced cellular leakage and suppressed oospore production of P.
aphanidermatum. Moreover, this study confirmed the efficient role of T. variabilis isolates in suppressing damping-off of cucumbers and tomatoes. Also, T. variabilis produced glucanase, cellulase and siderophores, suggesting the contribution of these metabolites in the inhibition of P. aphandermatum and its induced damping-off.
Cladosporium omanense inhibited growth of P. aphanidermatum and caused abnormalities in its hyphae. In addition, extracts form the fungus caused considerable cellular leakage and inhibited oospore production. In addition, bioassay test showed
that cucumber and radish damping-off diseases were significantly inhibited by C.
omanense, which may be attributed to its ability to produce cellulase, β-1,3- glucanases and siderophores.
The third and final part of the current study investigated the role of an endophytic fungus T. omanensis in promoting drought tolerance of tomatoes. T. omanensis provided multiple advantages to tomato under drought stress that included improving
reproductive characteristics, chlorophyll fluorescence and some anatomical characters such as increased phloem and cortex width and decreased stem hollow resulting from pith autolysis. In addition, T. omanensis significantly enhanced some
other important plant response parameters under drought stress such as increased shoot dry weight, root length, number of flowers and fruit weight. This study found that significantly higher concentration of gibberellic acid in tomato plants treated by
T. omanensis may have enhanced their drought tolerance.
The study showed that fungal isolates obtained from wild desert plants can have several advantages, including their potential use as antagonists against plant pathogens and to mitigate the effects of drought stress on some crop plants.
